In various systems using light such as of organism observation, a sensor, security, laser radar and the like, the technique of detecting desired signal light (light to be detected) is a fundamental and important element significantly influencing their performance. In particular, the needs for the high-speed high-sensitive detection technique are high.
For example, in organism observation, it is required to perform high-speed photodetection to enable accurate observation since the state and shape of the organism vary with time. Moreover, the organism is easily damaged by optical irradiation, and thus the amount of illumination light or excitation light with which an organism sample is to be irradiated has an upper limit. Therefore, the optical signal obtained from the organism is weak in general. For these reasons, the high-speed high-sensitive photodetection technique is strongly demanded in organism observation using light.
As the currently-used typical photodetection device, there can be mentioned a PMT (Photo Multiplier Tube), an APD (Avalanche Photo Diode) and a PD (Photo Diode). The PMT and the APD perform electron multiplying in the detection device, which enables high-sensitive photodetection. On the other hand, the PD does not have an electron multiplying function in the detection device and thus signals are usually amplified with the use of an electric amplifier, although it achieves a very high response speed. That is, any device of the PMT, the APD and the PD performs signal amplification electrically to improve the sensitivity.
Moreover, there can be mentioned, as the typical two-dimensional photodetector, a CCD (Charged Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), an EM-CCD (Electron Multiplying-CCD), an EB-CCD (Electron Bombardment-CCD) and an I-CCD (Intensified-CCD). When weak light is to be detected using the CCD or the CMOS, it is necessary, like the PD, to dispose an electric amplifier in a subsequent part so as to improve the sensitivity. The EM-CCD and the EB-CCD have an electron multiplying function in the detection device, like the APD, and achieve the higher sensitivity. The I-CCD has a configuration in which an Image Intensifier (I.I.) is disposed before the CCD. In the I.I, incident optical signals are converted to electrical signals once, and electron multiplying is performed in a MCP (Micro Channel Plate) embedded in the I.I., thereafter the multiplied electrons are rendered to collide with a fluorescent plate so that the multiplied electrical signals are converted to light again. The output light from the I.I. is converted to electrical signals by the CCD. That is, the I-CCD also performs signal amplification in an electric domain, thus achieving high-sensitive photodetection.
In the above conventional photodetection technique using signal amplification in an electric domain, there is a trade-off relation between the speed and the sensitivity, which makes it significantly difficult to achieve both high speed and high sensitivity. Therefore, it is unavoidable in the present situation that either of speed or sensitivity is sacrificed to perform photodetection.
As one of techniques capable of high-speed high-sensitive photodetection, the optical heterodyne detection technique is also widely used. The optical heterodyne detection technique is a photodetection method using interference effects by light to be detected and local light having an optical frequency slightly different from of light to be detected, and the intensity of local light is sufficiently increased so as to detect light to be detected with high sensitivity. Provided that the intensity of local light is sufficient, it is possible to perform ideal photodetection at a shot noise limit even when a high-speed electronic circuit is used. Thus, the high speed and the high sensitivity in photodetection are both achieved. In this case, however, it is general to use, as signal light and local light, lights enabling mutual interference to be stable temporally and spatially.
As a method for improving the temporal coherency, the following two methods are mainly used. The first method is a method in which the output from the same light source is split so that each is used as signal light and local light. In this case, the output of the light source is split, and thus it is arranged so that a relative delay time between signal light and local light before the recombination is shorter than a coherence time of the light source. Thereby, the interference state between signal light and local light becomes temporally stable. It is noted that the optical frequencies of signal light and local light are set to be slightly different from each other with the use of an optical frequency shifter or Doppler shift. This method has been conventionally used since the stable interference state can be achieved relatively easily (see Patent Documents 1, 2, for example).
The second method is a method using two independent light sources in which a line width of optical spectrum is significantly narrow (optical spectrum purity is significantly high) and an optical frequency is stabilized with high accuracy. These two independent light sources are used respectively as signal light or local light. In this case, the optical frequencies of signal light and local light are set to be slightly different from each other. This method has been conventionally very difficult to achieve because of technical constraints. However, since the recent technical development has made it possible to obtain a laser in which the optical spectrum purity is significantly high with a width of optical spectrum line being about kHz and an optical frequency is stabilized with high accuracy, the second method also enables a relatively stable interference state recently.
On the other hand, a spatial mode filter such as a confocal optical system or the like is used at the signal light side in order to improve the spatial coherency. Thus, only signal light components having the high coherency with local light are spatially extracted and used for optical heterodyne detection.
Patent Document 1: JP 06-21868 B2
Patent Document 2: JP 07-21452 B2